PROJECT SUMMARY Mucolipidosis type IV (MLIV) is an untreatable lysosomal storage disease caused by loss-of-function mutations in MCOLN1, gene encoding an endolysosomal cation channel TRPML1. MLIV patients display severe motor and cognitive disabilities. The main difficulty with MLIV treatment is that TRPML1 is completely lost in most MLIV patients. Thus, therapies based on stimulating residual TRPML1 activity are not likely to be effective. Because TRPML1 is not an endolysosomal enzyme, enzyme replacement and/or substrate reduction are not viable options. These barriers can be overcome by identifying and attenuating toxic signaling processes triggered by TRPML1 loss. Our preliminary data show transcriptional upregulation of the gene encoding ganglioside-induced differentiation-associated protein 1 (GDAP1) in the absence of TRPML1. Forced reduction of GDAP1 levels in cells lacking TRPML1 ameliorates the mitochondrial dysfunction that we have previously shown in MLIV. Given that GDAP1 expression is triggered by gangliosides ? lipids that accumulate in MLIV cells ? and that ganglioside accumulation has been shown in MLIV, we propose that gangliosides buildup in cell lacking TRPML1 triggers GDAP1 induction. We propose that the elevated GDAP1 in MLIV cells induces mitochondrial damage, vesicular trafficking defects, and neurodegeneration. Our model redefines the current view of MLIV pathogenesis, and suggests that suppressing GDAP1 expression and/or suppressing ganglioside production by pharmacological means may be used to mitigate tissue damage in MLIV. Here we combine the expertise of our labs in cell biology, Drosophila genetics and neurophysiology to test this model. In Aim 1, we will answer whether gangliosides and GDAP1 mediate the mitochondrial and vesicle trafficking defects observed in MLIV. The outcome of these experiments could confirm that extracellular gangliosides are a novel therapeutic target for MLIV. In Aim 2, we will test whether the loss of the TRPML1 homolog in Drosophila (trpml) leads to neurodegeneration via GDAP1-dependent mitochondrial dysfunction, using flies that lack trpml and recapitulate the key neurodegenerative features of MLIV. Specifically, we will examine whether GDAP1- dependent mitochondrial dysfunction and neurodegeneration occur in the Drosophila model of MLIV. We anticipate that our findings in the Drosophila MLIV model will inform future studies using human MLIV cells and vertebrate models. We will develop novel paradigms describing the mechanisms underlying mitochondrial dysfunction in an untreatable childhood neurological disease. Our approach, which bi-directionally combines in vitro cell physiology and the utility a Drosophila model, will allow us to make unprecedented progress to develop a deeper understanding of the mitochondrial dysfunction and neurodegeneration in MLIV.